In a gas turbine engine, inlet air is continuously compressed, mixed with fuel in an inflammable proportion, and then contacted with an ignition source to ignite the mixture which will then continue to burn. The heat energy thus released then flows in the combustion gases to a turbine where it is converted to rotary energy for driving equipment such as an electrical generator. The combustion gases are then exhausted to atmosphere after giving up some of their remaining heat to the incoming air provided from the gas turbine compressor.
The starting of a gas turbine engine is a complex operation which is most desirably done as simply and as automatically as possible. Typically, before it is run on its own power, the gas turbine engine must be accelerated by an external source of power to provide sufficient air flow to the combustor for light-off. Typically, engine speed varies as a function of the torque versus speed characteristics of the starter motor. Fuel flow to the gas turbine engine is scheduled "open loop" as a function of engine speed and ambient conditions such as temperature and atmospheric pressure. This "open loop" approach is, however, extremely sensitive to fuel control variations, gas turbine engine variations, sensor accuracy and ambient conditions to achieve the proper or correct fuel-to-air ratio required for ignition and subsequent combustion. Starting the gas turbine engine at low temperatures and high altitudes is particularly troublesome with this "open-loop approach.
In addition, improved performance requirements for a gas turbine engine and increasing restrictions on gas turbine engine emissions have led to gas turbine engine designs with improved atomization of the fuel and increased mixing of the fuel and air such that the fuel-to-air ratio is maintained relatively lean. What this means is that the ignition envelope, that is the envelope or fuel-to-air ratio where ignition is possible, is highly dependent on the local velocity, and the pressure and temperature of the gases. This ignition envelope is thus significantly narrowed and thus is harder to achieve.